文摘
We have studied gas adsorption effects on the structure of a metal鈥搊rganic framework, MIL-53(Al), a material well-known because of the controllable framework 鈥渂reathing鈥?phenomenon. Neutron powder diffraction between 4 and 77 K and up to 4.5 bar pressure D2 confirms that a structural phase transition is responsible for the observed H2/D2 isotherm hysteresis at 77 K. We find two crystallographically distinct D2 adsorption sites in MIL-53(Al) when the pores are fully opened, similar to those reported for D2 in MIL-53(Cr), but in contrast with the previously published cases of CO2 and H2O. Upon desorption of D2 at 77 K, we find strong evidence for the existence of D2 molecules kinetically trapped in the center of the closed pore of MIL-53(Al). This 鈥渕olecular clamp鈥?appears to be functional until 鈮?20 K, the temperature at which the D2 eventually desorbs under dynamic vacuum. Hydrogen diffusion constants calculated using quasielastic neutron scattering data collected along the isotherm are also consistent with H2 being trapped in the closed pore structure. Diffraction experiments performed with N2 and He gases under similar conditions show the range of structural response from immediate pore opening at low N2 pressures (<1 bar) to no observable effect at 10 bar He.